I started insanity workout and finished the sixth week and stopped for some reasons. A month later I decided to start again and did the fit test, which was really painful with a state of mind of reaching the same level as my last fit test before I stopped. I eventually could reach the same level. Then I continued the insanity workout starting directly at the sixth week. I couldn't do it continuously and had to stop at many points because of strong dizziness. Skipping some 45 seconds cycles I was able to finish it. My question is if those dizziness periods are dangerous and what are the causes behind them? lack of oxygen? fatigue? or just a heart that is not powerful enough to keep up with the pace?
Dizziness in training can come from a variety of factors. Most everyone who's lifted very heavy doing compound barbell lifts has felt a little woozy after a few reps of heavy weights, and you'll hear a lot of endurance athletes talking about "almost passing out". But real fainting (syncope) in exercise is serious:
Syncope is a common event in which there is a transient loss of consciousness and postural tone. Although syncope is generally a benign event in young adults (less than 35 years of age) and, in many cases, never reaches the attention of a physician, exercise-related syncope can signal sudden death.
Vasovagal response, what you could be experiencing, basically means that the vagus nerve has messed up a bit in regulating blood pressure and supply to the brain. The causes of vasovagal response are wide: ranging from standing up too fast to dehydration to being exposed to strong magnetic fields.
You should talk to a physician, to have them rule out anything serious. This isn't blanket advice: I personally know of an extremely fit athlete who had serious cardiac damage because he developed undiagnosed problems in life and attributed his symptoms to general fatigue.
Talk to your normal doctor but consider talking to a sports medicine physician who probably has the equipment in his or her office to conduct the necessary stress test.
As stated in the other answer, there isn't really any way to diagnose why this happened.. however in my experience I've suffered from feeling dizzy to the point of almost collapsing but only after very serious workouts with heavy kettlebells and minimal rest periods between sets. In my experience it generally boils down to lack of blood sugar pre workout but obviously hydration is also important.
If you're a fit person and have never had any heart problems (or anything else) then it's probably just what I've described however you should seek professional medical advice as we don't want to risk killing ourselves do we?